Shaded pole electric motors have been used extensively for many years for control purposes. These shaded pole motors utilize shading windings on shading poles and by opening or short circuiting the appropriate shading windings, the necessary shading action to cause the motor to rotate either in a clockwise or counterclockwise direction is developed. This type of motor normally has not been provided with any type of electromechanical brake, but has been used in modulating control applications.
Some control motors have been used with brakes where the brake action is developed by the armature of the motor shifting in the magnetic structure to center the rotor. This centering action is used to energize or de-energize a brake to lock the motor in the de-energized position. In shaded pole electric motors of larger sizes, this type of brake action has been inadequate or impractical.
Dynamic braking has been utilized with this general type of motor, as well as other electric motors, wherein a direct current potential is applied to the motor winding when the motor is de-energized. This normally requires a separate source of direct current potential and some type of switching arrangement to determine when the motor is de-energized. One of the simpler arrangements that has been used in the past is to charge a capacitor from a rectifier source connected to the energizing source for the electric motor, and then apply the capacitor to discharge through the motor winding to brake the motor operation to a stop. The use of a capacitor discharge does not, however, hold the motor at the braked position but merely aids in stopping the motor. The application of continuous direct current voltage to the motor winding has been provided in prior art arrangements from a direct current source, but this arrangement is very expensive in small control motors.